Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into mechanical energy and then subsequently converts the mechanical energy into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle. The rotor is coupled with a generator for converting the kinetic energy of the blades to electrical energy.
Wind turbines are massive machines having relatively large masses moving at relatively high rates of speed. For example, the rotor of a modern wind turbine may weigh in the range of 25-50 tons and have blade tip speeds around 300 ft/s. Additionally, generator components, such as the rotor assembly which typically carries heavy magnets or the like, have considerable weight and are subject to relatively high rotational speeds. Thus, it may be important to incorporate within a wind turbine measures or systems configured to control these components, and more specifically, for reducing the rotational speed of these components under certain conditions.
Conventional wind turbine designs may provide a number of ways to reduce the speed of the rotor and generator of the wind turbine. For example, many modern wind turbines include blade pitch mechanisms that allow the blades to rotate about their longitudinal axis to affect the aerodynamic forces acting on the blades. The pitch mechanisms may be used to pitch the blades, for example, out of the wind so as to slow the wind turbine rotor and generator. Thus, for example, when wind conditions become high or excessive, the blades may be pitched in order to reduce the lift forces acting on the blades, and thus reduce the speed of the rotor and the generator operatively coupled thereto. In a further example, in a grid fault, the electrical load on the generator drops suddenly, thereby causing the generator speed and rotor speed to suddenly increase. In these over-speed conditions, the blades may again be pitched in a manner that reduces the rotor and generator speeds.
In addition to pitch mechanisms, wind turbines may also include other braking mechanisms configured to reduce the speed of the rotor or prevent the rotor from turning. In this regard, wind turbines may include mechanical braking systems that rely on friction between two surfaces (e.g., rotor disc and pads) to reduce or restrict the rotation of the rotor. For example, various drum and disc brake systems have been used in various wind turbine arrangements to reduce the speed of the rotor and/or to secure the rotor in a parked position.
These braking systems, however, are not without their drawbacks. In this regard, pitch-based rotor braking may impose stresses in other wind turbine components, such as the wind turbine tower or foundation, for example. Additionally, friction-based rotor brakes require regular maintenance and replacement parts, including disc and pad replacement that are subject to wear and damage. Moreover, friction-based rotor brakes are primarily effective once the rotational speed of the wind turbine rotor has already been significantly reduced. Thus, these types of brakes may not be particularly useful under certain high speed conditions where it is desired to reduce the speed of the rotor.
Accordingly, there is a need for a braking system that addresses these and other shortcomings of existing wind turbine braking systems. More particularly, there is a need for a braking system that reduces or eliminates the need for regular maintenance and replacement parts, can be used over a relatively large range of rotor speeds, and minimizes the impact of a braking procedure on other or adjacent wind turbine components.